1. Field of the Invention
The present invention relates to a crystal oscillator, and more particularly, to a crystal oscillator that can increase the degree of freedom in design of vibrators and oscillators and readily achieve desired characteristics.
2. Description of the Related Art
Referring to FIG. 3, the structure of a conventional crystal oscillator is described. FIG. 3 is a schematic cross-sectional view showing the structure of a conventional crystal oscillator.
As shown in FIG. 3, an IC 7a and electronic components 7b are housed in a concave portion formed in a ceramic package 1, and a vibrator 4 is placed over the IC 7a and the electronic components 7b. 
In the crystal oscillator having the structure, the vibrator 4 is fixed to step-like supporting portions in the ceramic package 1 with a conductive adhesive agent 6. Further, a sealing ring 3 for tightly sealing the case is provided at the upper portion of the ceramic package 1, and a cover 2 is placed on the sealing ring 3, so that the package is tightly sealed.
As described above, in the conventional crystal oscillator, the IC 7a and the electronic components 7b are placed under the vibrator 4. Therefore, the vibrator 4 needs to be large enough to secure a space for housing the IC 7a and the electronic component 7b. Here, the degree of freedom in designing the vibrator 4 is low, and there is a high possibility that the desired characteristics cannot be achieved.
Also, since the IC 7a and the electronic components 7b are housed under the vibrator 4, the vibrator 4 is likely to be affected by the heat released from the IC 7a and the electronic component 7b, and the oscillation frequency might become unstable.
Referring now to FIG. 4, another conventional crystal oscillator is described. FIG. 4 is a schematic top view of another conventional crystal oscillator.
As shown in FIG. 4, in this crystal oscillator, an IC 7a and electronic components 7b are housed in a concave portion in a ceramic package 1. A pedestal 5 made of crystal is provided to cover almost half the opening of the concave portion, and a vibrator 4 is placed on the pedestal 5.
The pedestal 5 is formed with a crystal plate made of the same material as the vibrator 4. The pedestal 5 has no involvement in oscillations, and is one size larger than the vibrator 4. The pedestal 5 is placed on supporting portions 8e, 8f, and 8g formed in the ceramic package 1, and is fixed onto the supporting portions 8e, 8f, and 8g with a conductive adhesive agent.
The supporting portions 8e, 8f, and 8g have step-like shapes in the ceramic package 1.
Although not shown, the vibrator 4 is connected to a wiring unit formed on the side of the bottom face via an electrode. The IC 7a is connected to the electronic components 7b by wire bonding.
With this arrangement, even if the vibrator 4 is small in size, it is possible to secure a space large enough to house the IC 7a and the electronic components 7b. Also, as the pedestal 5 absorbs the heat released from the electronic components 7b, the temperature variation of the vibrator 4 can be made smaller, and excellent frequency characteristics can be achieved.
Conventional techniques relating to crystal oscillator structures are disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 2007-97040, 11-355047, 2005-033292, and 2005-292079, for example.
JP-A No. 2007-97040 discloses a piezoelectric vibrator in which two or more piezoelectric vibrator plates are housed in a housing, and excitation electrodes are formed on the respective plates and are arranged in a non-overlapping manner in the housing.
JP-A No. 11-355047 discloses a piezoelectric oscillator that has a crystal vibrator placed on a first step-like portion formed in a package, and has a single-layer or multi-layer ceramic cover placed on a second step-like portion.
JP-A No. 2005-033292 discloses an electronic component package that has a concave portion having an upward opening and a concave portion having a downward opening, and allows crystal vibrators to be placed in the respective concave portions. This patent publication also discloses a piezoelectric device that uses the electronic component package.
JP-A No. 2005-292079 discloses a piezoelectric device in which a crystal vibrating piece is held by lead wires, the lead wires form a space between the crystal vibrating piece and a supporting substrate, and a circuit element for driving the vibrating piece is provided in the space.
In the other conventional crystal oscillator, however, the supporting portions 8 having the pedestal 5 mounted thereon in the package protrude from the inner walls of the package. Particularly, the supporting portions 8e and 8g cause a problem by hindering wire bonding connections between the IC 7a and the electronic components 7b. As the crystal oscillator is made smaller in size, this problem with the supporting portions at the time of wiring becomes more serious.